1. Field of the Invention
This invention relates to an improved apparatus and process used in the operation of down-flow granular medium bed filters.
2. Information Disclosure Statement
Down flow granular medium bed filters rely on a bed of granular material to filter solids from a waste liquid. The bed, of course, must maintain its integrity during filtration to prevent solids from passing through the filter. Commonly, a facility practicing granular medium filtration will employ a number of filter cells in its design. Each filter cell can operate independently of the other cells which allows one or more cells to be backwashed or receive servicing while the remaining cells continue to filter waste liquid.
Each filter cell receives waste liquid through an inlet. Most commonly an inlet trough, located centrally within a cell and a short distance above the filter bed, distributes influent liquid over the bed within the cell. The U-shaped influent trough generally has sides 12 to 18 inches high and influent liquid overflows the trough onto the filter bed. To prevent the influent liquid from disturbing the granular medium bed, splash plates are positioned a short distance above the bed to dissipate the force of the falling influent from the top of the trough. For a centrally located rectangular trough within a filter cell, a rectangular splash plate is positioned on each side to intercept influent trough overflow and dissipate the force of the falling liquid. The splash plates are normally fastened to support means to secure them in place and render the plates stationary. The plates are generally made of aluminum or fiber glass reinforced plastic to avoid corrosion problems. Some filter cells may be configured with the inlet trough located at one side of the cell such that influent liquid overflows only one side of the trough onto a single splash plate. Other cells may have a simple pipe inlet which directs influent onto a splash plate, although this configuration is less common.
In U.S. Pat. Nos. 3,459,302, Re. 28,458, 3,516,930, 3,587,861 and 3,817,378 Ross discloses an influent trough with stationary splash plates on either side to dissipate energy from the liquid overflowing the trough. Ross also discloses in U.S. Pat. No. 4,627,923 a hollow rectangular pipe which acts as a splash plate for influent liquid overflowing the inlet trough. Also disclosed is the mixing of oxidizing agent and surfactant within the filter bed by introducing each substance by separate conduit systems below and within the filter bed.
Hadden et al. in U.S. Pat. No. 3,933,641 employ a splash pan with a trough, baffles and holes to distribute effluent over the surface of an underlying filter bed. The splash pan is supported above the filter by brackets on the filter walls and the pan covers the whole surface of the filter tank for evenly distributing effluent over the whole surface of the filter bed.
Scholten et al. in U.S. Pat. No. 4,076,625 disclose a granular-media filter with wash trough and baffle assembly to reduce media loss during high-turbulence washing of the media. The baffle is described as having an upwardly concave arcuate portion in opposed relation to the trough lower portion, and defining a restricted flow channel for wash water to the trough.
In U.S. Pat. No. 4,366,058 Wolde-Michael discloses a multicompartmented settling tank with a variously adjustable solid plate which is normally inclined at a 45.degree. angle. The plate controls fluid flow rate and also acts as a floating material trap along its lower portion and as a settling area for fine particulate matter at its upper portion.
The inlet trough of a granular medium bed filter may also function as a collector for the removal of backwash liquid and the solids flushed from the bed during backwashing. In the backwashing cycle, liquid flows up through the bed, carrying filtered particles with it into the trough and out of the filter cell to further treatment or disposal.
The inlet/backwash trough and splash plates can cover a significant portion of the filter bed surface and this may impede filtration, backwashing or chemical cleaning of the filter cell. The splash plates may shield the portion of the filter bed directly below them from receiving an even distribution of particulates from the influent liquid. During backwashing, the splash plates may interfere with the scouring action of the granular medium which helps clean the medium. Likewise, when an air outlet below the liquid surface is used to creates currents in the liquid above the filter bed and sweep particles into the trough during backwashing, the splash plates may interfere with the circulation currents which carry particles into the trough. Addition of cleaning chemicals to the filter bed from above is also impeded by the splash plates which shield that portion of the bed directly below the plates from the cleaner. Therefore, we have devised a splash plate configuration which overcomes all of these drawbacks discussed above.